Hollow fiber membrane-based dehydration is known. See, for example, U.S. Pat. Nos. 4,783,201, 4,725,359, 4,718,921, 4,497,640, 4,583,996 and 3,511,031. U.S. Pat. No. 4,793,830 discloses a process for removing small amounts of water and carbon dioxide from compressed air to generate ultrapure instrument quality gas with silicone-coated hollow fibers. However, the water vapor permeability and selectivity of the membranes of this patent are far less than those for the membranes of the present invention, resulting in far less efficient dehumidification. U.S. Pat. No. 4,900,626 discloses a hollow composite fiber for dehydration having a polydimethylsiloxane coating on a dense layer of the fiber support, the support thus having relatively low permeability.
For a variety of reasons, no known prior art approach has been able to achieve a high degree of efficiency while maintaining a low-cost, simple design. The reason for this appears to be that, while the theory of gas dehydration is ostensibly simple, in practice the task often proves to be quite complex due to an unusually large number of factors that come into play when hollow fiber separation membranes are used, such as the resistance, selectivity and permeability of the permselective layer, the resistance, selectivity, and permeability of the support fibers, and the resistance of the module. In addition, the relative values of these resistances and the directions of feed and permeate and sweep gases also impact the fluxes of the various permeating species, and thus, the overall performance of the module.
It is therefore an object of the present invention to provide a highly efficient method and apparatus for gas dehydration.
It is a further object of the present invention to provide a low-cost and easy-to-operate gas dehydration module.
It is a still further object of the present invention to provide a novel, highly selective composite hollow fiber membrane.
These and other objects, which will become apparent to one of ordinary skill, are accomplished by the present invention, which is summarized and described in detail below.